The invention relates to devices for illuminating objects, particularly to devices for efficiently illuminating a distant object for a prolonged length of time despite index unhomogeneities separating the illuminating device from the object.
To illuminate a distant object it is advantageous to use coherent optical radiation, emitted by a laser for example. But the propagation of a laser beam in the atmosphere is generally accompanied by considerable disturbance of the phase of the optical wave. These distortions are created by the presence, in the atmosphere, of random index gradients or by the passage through optical components presenting an unhomogeneous distribution of the refraction index. Thus, the presence of disturbances prevents a maximum of radiated energy from reaching the object to be illuminated.
To palliate these disadvantages, the illuminating device of the invention uses means for ensuring good concentration of radiated energy on an object to be illuminated for a prolonged period of time, from an initial transitory illumination, whatever the turbulences and disturbances in the atmosphere traversed.
This result is obtained by using, for constructing the illuminating device, the so-called four-wave mixing techniques. According to these techniques, by causing, in a photoexcitable optical medium with index variation, an incident wave with any wave front to interfere with a pump wave, there is generated in real time a conjugate wave of the incident wave, i.e. a wave which follows exactly the same path, in the reverse direction, as the incident wave and so undergoes the same deformations in a reverse direction.